In a magnetic media, digital information (expressed as combinations of “0's” and “1's”) is written on tiny magnetic bits (which themselves are made up of many even smaller grains). When a bit is written, a magnetic field produced by the disk drive's head orients the bit's magnetization in a particular direction, corresponding to either a 0 or 1. The magnetism in the head in essence “flips” the magnetization in the bit between two stable orientations.
A plot of the magnitude of magnetization or flux density as a function of applied magnetic field is called a hysteresis loop. A typical hysteresis loop is shown in FIG. 1. In this case, a magnetic field H has been applied to a sample of magnetic material, and the component of magnetization in the direction of H has been plotted. Such a loop could be directly plotted, for example, by an instrument often used in the analysis of magnetic materials called a vibrating sample magnetometer (VSM). The VSM mechanically oscillates a magnetic sample in a static magnetic field, measuring the component of M in the direction of H as H is slowly varied in magnitude.
Another common method of producing hysteresis loops is by placing the sample in a long solenoid. The solenoid creates a rapidly oscillating magnetic field H. The sample flux density is read from a small pickup loop placed near the sample. Since even without a sample, the pickup coil will pick up the field of the solenoid, two identical pickup coils are used with the sample mounted inside one of them. The signal is the difference between the output of the two coils.
In FIG. 1, starting in zero H field with a demagnetized sample, the field is gradually increased in the positive H direction. The path the magnetization initially takes, the initial magnetization curve, is 0-a-b-c. The initial slope is called the initial susceptibility χi. If one was plotting B versus H instead of M versus H, the initial slope would be called the initial permeability. The maximum value of magnetization reached is called the saturation magnetization, Ms, where application of additional H will yield no appreciable increase in M. When the field is now reduced and reversed, the magnetization will take the path around the loop labeled c-d-e-f-c. The value of M at zero field on this major loop is called the remnant magnetization Mr. The value of H for zero magnetization on this major loop is called the coercive force or coercivity Hc. The early part of the initial magnetization curve from 0 to a is nearly linear and is reversible. The next section, from a to b, is irreversible. The final section from b to c is reversible again. These features of the hysteresis loop depend on the domain wall motion and magnetization rotation.
Ferromagnetic materials are divided into two broad classes: soft magnetic materials and hard materials. Hard magnetic materials show low initial permeability (or susceptibility) and high coercive force. FIG. 1 shows a hysteresis loop that is characteristic of a hard magnetic material that might be used for disk media or for a permanent magnet application. Soft magnetic materials exhibit high initial permeability (or susceptibility) and also low coercive force. FIG. 2 shows a hysteresis loop that is characteristic of a soft magnetic material that might be used for a transformer or a magnetic head application.
FIG. 3 shows values for permeability and coercive force for some representative magnetic materials. Supennalloy and 78 Pennalloy may be considered soft magnetic materials, and Alnico V and ferroplatinum may be considered hard magnetic materials. This figure shows a rather clear inverse relationship between penneability and coercivity. It also should be noted that both of these parameters vary over a remarkably large range-five to six orders of magnitude.
The ease of magnetization or demagnetization of a magnetic material depends on the crystal structure, grain orientation, the state of strain, and the direction of the magnetic field. The magnetization is most easily obtained along the easy axis of magnetization but most difficult along the hard axis of magnetization. A magnetic material is said to posses a magnetic anisotropy when easy and hard axes exist. On the other hand, a magnetic material is said to be isotropic when there are no easy or hard axes. A magnetic material is said to possess a uniaxial anisotropy when the easy axis is oriented along a single crystallographic direction, and to possess multiaxial anisotropy when the easy axis aligns with multiple crystallographic directions.
“Anisotropy energy” is the work against the anisotropy force to turn magnetization vector away from an easy direction. For example, a single crystal of iron, which is made up of a cubic array of iron atoms, tends to magnetize in the directions of the cube edges along which lie the easy axes of magnetization. A single crystal of iron requires about 1.4×105 ergs/cm3 (at room temperature) to move magnetization into the hard axis of magnetization from an easy direction, which is along a cubic body diagonal.
Important magnetic properties, such as coercivity (Hc), remnant magnetization (Mr) and coercive squareness (S*), which are crucial to the recording performance of the Co alloy thin film depend on the degree of uniformity of the composition and microstructure of the film. Particularly, in high-density recording media, an important bulk property is coercivity (Hc) besides remnant magnetization-thickness product (MrT), where Mr is the remnant magnetization and T is the film or layer thickness. (In this application, “T” refers to thickness, not temperature.) With the rapid growth in recording areal density, the uniformity of Hc throughout the disk is desired because any non-uniformity could cause degradation in the read-write performance of the finished magnetic media, which in turn could affect the product yields at both media component level, and at the finished drive product level. For further improvement of the magnetic properties, this invention proposes a novel process for fabricating novel disk media having substantially uniform Hc throughout the disk media.